Missing error message or timeout, while using FederatedX engine and the remote database cannot be reached

For about 2 days now, most queries on the information_schema tables do not bring any results within 10 minutes. Queries from Nagios sometimes hang longer than 6 hours in the process list.
The queries from Nagios usually run within one second.

Even the query "select * from information_schema.key_column_usage limit 1;" does not bring any result within 10 minutes.

The only change made on the database server is the installation of the Sentinel virus scanner. But even temporarily disabling Sentinel did not bring any improvement in the runtime of the above query.

What can I do to find out why the above query is running so long?